feat: device-security nudge — verify this device / key backup (#110, #123)

One "security" strip, in the same top slot and style as the status banner
(#124), for this device, in priority order:
- "Verify this device" — cross-signing exists but this device isn't
  verified (can't unlock backed-up history, untrusted to others). First,
  because verifying with the recovery key also connects the backup.
- "Connect this device to your key backup" — a backup exists, this device
  isn't using it.
- "Set up key backup" — no backup at all (includes accounts without
  cross-signing; the setup flow does both).
The button opens Settings → Devices (existing flows); "Not now" snoozes.

Rules: nothing in a device's first 24 h; "Not now" snoozes that nudge 7
days; 3 dismissals stop it; never on a healthy device; waits until sync
has settled (never under "Connecting…"); outage/maintenance/connection
strips win. Per-device record in localStorage, wiped on logout. Mounted
inside the Matrix client context and an error boundary (an early version
outside the context crashed the app on load — caught before pushing).

Also: the status strip wraps its text on phones instead of truncating it
when there's no Details button (benefits #124's strips too).

Design approved on #123 (real-client screenshots there). Tests: 4 unit
(the #123 state table, loading, timing, stored record); e2e: nothing during
the grace period, "Set up key backup" → Settings → Devices, "Not now"
holds across a reload. Unit 1319, Playwright 29 passed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/utils/securityNudge.ts

@@ -0,0 +1,94 @@
+/**
+ * [Gitea #110, #123] One "security" nudge for this device, in priority order:
+ *
+ * - verify-device: the account has cross-signing but THIS device isn't signed.
+ *   It can't unlock backed-up history and shows as untrusted to others.
+ *   Verifying with the recovery key also connects the backup, so it comes first.
+ * - connect-backup: a key backup exists on the server but this device isn't
+ *   backing up to it (`getActiveSessionBackupVersion()` null).
+ * - setup-backup: no key backup at all (includes accounts that never set up
+ *   cross-signing: the setup flow does both).
+ *
+ * Anything not yet known (crypto still loading) → no nudge. Timing: not in
+ * the first 24 h on a device (onboarding), "Not now" snoozes 7 days, three
+ * dismissals of a nudge stop it for good; a healthy device never sees one.
+ */
+
+export type NudgeKind = 'verify-device' | 'connect-backup' | 'setup-backup';
+
+export type SecurityState = {
+  /** m.cross_signing.master account data present. */
+  crossSigning: boolean;
+  /** This device cross-signing-verified; undefined while crypto isn't ready. */
+  deviceVerified: boolean | undefined;
+  /** A key backup version on the server; undefined while loading. */
+  backupOnServer: boolean | undefined;
+  /** This device is backing up to it; undefined while loading. */
+  backupActive: boolean | undefined;
+};
+
+export const nudgeFor = (s: SecurityState): NudgeKind | undefined => {
+  if (s.crossSigning) {
+    if (s.deviceVerified === undefined) return undefined;
+    if (!s.deviceVerified) return 'verify-device';
+  }
+  if (s.backupOnServer === undefined) return undefined;
+  if (!s.backupOnServer) return 'setup-backup';
+  if (s.backupActive === undefined) return undefined;
+  return s.backupActive ? undefined : 'connect-backup';
+};
+
+export const NUDGE_GRACE_MS = 24 * 60 * 60 * 1000;
+export const NUDGE_SNOOZE_MS = 7 * 24 * 60 * 60 * 1000;
+export const NUDGE_MAX_DISMISSALS = 3;
+
+export type NudgeRecord = {
+  /** When this device was first seen by the nudge (ms). */
+  firstSeen: number;
+  dismissals: Partial<Record<NudgeKind, { count: number; last: number }>>;
+};
+
+export const shouldShowNudge = (kind: NudgeKind, record: NudgeRecord, now: number): boolean => {
+  if (now - record.firstSeen < NUDGE_GRACE_MS) return false;
+  const d = record.dismissals[kind];
+  if (!d) return true;
+  return d.count < NUDGE_MAX_DISMISSALS && now - d.last >= NUDGE_SNOOZE_MS;
+};
+
+export const dismissNudge = (kind: NudgeKind, record: NudgeRecord, now: number): NudgeRecord => ({
+  ...record,
+  dismissals: {
+    ...record.dismissals,
+    [kind]: { count: (record.dismissals[kind]?.count ?? 0) + 1, last: now },
+  },
+});
+
+export const NUDGE_KEY_PREFIX = 'lotus-security-nudge-';
+
+/** Parse a stored record; anything unexpected starts a fresh one (grace period from now). */
+export const parseNudgeRecord = (raw: string | null, now: number): NudgeRecord => {
+  try {
+    const v = JSON.parse(raw ?? '');
+    if (v && typeof v.firstSeen === 'number' && v.dismissals && typeof v.dismissals === 'object') {
+      return { firstSeen: v.firstSeen, dismissals: v.dismissals };
+    }
+  } catch {
+    /* fresh record below */
+  }
+  return { firstSeen: now, dismissals: {} };
+};
+
+export const NUDGE_COPY: Record<NudgeKind, { text: string; action: string }> = {
+  'verify-device': {
+    text: 'Verify this device so you can read your older encrypted messages and others see it as trusted.',
+    action: 'Verify',
+  },
+  'connect-backup': {
+    text: "This device isn't using your key backup yet. Connect it so your encrypted messages stay readable if you lose it.",
+    action: 'Connect',
+  },
+  'setup-backup': {
+    text: "Your encryption keys aren't backed up. Set up key backup so you don't lose your encrypted messages if you log out or lose this device.",
+    action: 'Set up',
+  },
+};